This commit also fixes a glitch in Qt5 where the outline of some
selected TilemapViewer cells were not rendered correctly if the zoom was
set to a prime number.
Qt5's default pen width is 1.0 and therefore the grid line thickness
increases when zooming in.
This commit changes the grid pen width to the inverse of zoom so it is
rendered same thickness independent of the zoom setting.
Optiroc has mentioned that the selected tile box's position and size is
multiplied by two on his Mac. This is caused by either
`painter->combinedTransform()` returning the wrong value for hi-dpi
displays or `painter->resetTransform()` not working as expected on
hi-dpi displays.
The new code does not call `painter->resetTransform()` but instead sets
the pen width and cell adjust to the inverse of the zoom, which cancels
out the transform and should render as one pixel to the user.
